This invention is an improvement of a known book-type container for raising seedlings. The container is disclosed in Canadian patents 989,614 and 1,068,482 issued to the present applicant.
In general, the container referred to comprises opposed front and rear wall members hinged together along their base. The wall members can be pivoted about their lower hinge from a closed, vertical, pressed together position to an opened flat position. The container opens and is held like a book and is referred to as a book-type seedling container. Each wall member has spaced apart, inwardly projecting elongate shoulders joined by connecting webs. Mutually opposed pairs of shoulders come together in the closed position to complete, in conjunction with the webs, the formation of vertical, elongated, open-topped cells or cavities. At their lower or base end, the webs and shoulders forming a cell define a bottom opening. This opening functions to air-prune the downwardlY projecting roots of a seedling being raised in growing medium filling the cavity.
Each pair of opposed shoulders is formed to provide a shiplap joint. This is done to reinforce the container against lengthwise slippage of one wall member relative to the other. If such slippage takes place then the effectiveness of the seal, provided by the shoulders forming the two side walls of the cell, is lost. Once the seal is lost, the roots from one cell will penetrate through the breached seal into the next cell. This creates serious difficulties as the root systems of the separate seedlings are now entwined.
The known shiplap joint is illustrated in FIGS. 10 and 12 of Canadian Patent 1,068,482. It involves providing shoulders on one wall member which are formed by low and high ledges extending in parallel adjacent relationship. The ledges are of about equal width. Close to the outer end of the shoulder, there is a "crossover"--that is the low and high ledges exchange positions for the balance of the length of the shoulder. As a result of its stepped configuration, the shoulder forms a long recess extending along one side of the shoulder and then, at the crossover point, a short recess extending the balance of the length of the shoulder along its other side. The opposing shoulder on the other wall member is formed in the same manner except that the long and short recesses are formed along sides opposite to those of the first shoulder, when the two shoulders are pressed together, the high ledges of the shoulders seat in the recesses of the opposite shoulder, to form a solid wall between cells. This solid wall has a part-line, but that part-line is tortuous in configuration and resists root penetration. The "crossover" structure of the two shoulders functions to resist longitudinal displacement of the container walls.
This shiplap joint has long worked reasonably effectively for its purpose. There was a weak spot, at the crossover, where root penetration Would occasionally take place. But the crossover surfaces could be formed precisely enough with the stiff plastic sheet heretofore used, to preclude most root penetration.
However, the weakness at the crossover has been found to be more severe when more flexible plastic sheet (such as that obtained from recycled plastic) is used to fabricate the container. There is therefore a need to improve the seal.